None at all. But if you want a botanically precise person to know what
you are talking about, why not use Horsetail and Mare's-tail to indicate
two completely different plants?

OK. here's the year 2000 revision of the OED entry for Marestail

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Â*Â*1. a. The aquatic flowering plant Hippuris vulgaris (family
Hippuridaceae), having an erect stem with leaves in whorls and small
greenish axillary flowers; formerly also called female horsetail. Also:
any of several horsetails (genus Equisetum), which Hippuris vulgaris
superficially resembles. Cf. HORSETAIL n. 2.
?a1425 MS Hunterian 95 f. 194v, Cauda equina: Maire tayle or schaue
gresse. 1526 Grete Herball ccxx. sig. Niii/2, Iperium is an herbe that
is called mares tayle. 1762 W. HUDSON Flora Anglica 2 Hippuris..Anglis
Mare's-tail. 1785 T. MARTYN tr. J. J. Rousseau Lett. Elements Bot. xi.
122 (note) In the books it [sc. Hippuris] is called Female Horse-tail or
Mare's-tail. 1864 TENNYSON Aylmer's Field 92 The petty marestail forest,
fairy pines. 1868 SIR J. HOOKER Addr. in Rep. Brit. Assoc. Advancem.
Sci. p. lxv, The existing family of Equisetaceæ..contained previously
but one genus, that of the common mare's tails of our river-banks and
woods. 1879 R. JEFFERIES Wild Life 374 On the shore, where it is marshy,
the mares-tail flourishes. 1919 W. GRAVESON Brit. Wild Flowers (ed. 2)
xxiii. 217 Some of the old writers regarded the plant [sc. Hippuris] as
the female of one of the equisetums, and hence the transition of name
from Female Horse-tail to Mares-tail. 1960 S. ARY & M. GREGORY Oxf. Bk.
Wild Flowers 52/2 Mare's-tail is a widespread but only locally common
plant of lakes, streams, and ditches. 1990 Gardening from ‘Which’? Aug.
272/3 Casoron G4 can be used to kill mare's tails. 1991 C. STACE New
Flora Brit. Isles 11 The common name Mare's-tail is often used [of the
genus Equisetum] but is a misapplication; true Mare's-tail is Hippuris.
Â*Â*Â*Â*b. U.S. regional. Horseweed (Canadian fleabane), Conyza canadensis.
1900 A. B. LYONS Plant Names 221 L[eptilon] Canadense... Mare's-tail.
1935 W. C. MUENSCHER Weeds 483 Mares-tail... Common throughout North
America. 1993 T. COFFEY Hist. & Folklore N. Amer. Wildflowers 255/1
Horseweed. Conyza canadensis... Bittersweet,..Canada
Fleabane,..Mare's-Tail, [etc.].
Â*Â*Â*
Â*2. A long straight streak of cirrus cloud, often believed to herald
stormy weather. Usu. in pl. exc. in attrib. use, as mare's tail cirrus,
cloud.

Â*Â*Â*Â*3. Anat. = cauda equina s.v. CAUDA n. Obs. rare0.
1890 Cent. Dict., Mare's-tail,..in anat., the cauda equina.


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From which I conclude that when the word "marestail' is used to
refer to a plant, it is either hippuris vulgaris, any of several
horsetails (genus Equisetum) or conyza canadensis or it is a usage
as yet unrecorded by the OED.
I would expect any botanically precise person, who wished the reader
to understand what was being referring to, to use the specific
latin name. In looser circumstances, context is used to
decide. Marestail + obnoxious weed + allotment + UK = equisetum sp.